The present invention is directed to a virtual imaging system, and more particularly, to a virtual imaging system suitable for use in a head-mounted imaging system.
A virtual imaging system is a system in which a user views a virtual image of a display (or object) rather than the physical display itself. A typical virtual imaging system including a display, hereinafter referred to as xe2x80x9cvirtual displayxe2x80x9d, is shown in FIG. 1 in which a user""s eye 2 looks through a lens 4 and sees a virtual image 6 of a physical display 8.
In a virtual display, it is possible to create the appearance of a large display at a comfortable viewing distance from a user""s eye. Recent developments have been made in microdisplays such that XGA (extended Graphics Array) computer screens can be made at lower cost on silicon chips having an area of approximately 1 cm2. It is highly desirable to provide a small virtual display system that can take advantage of such a microdisplay in various applications, such as a head-mounted, hand-held, body-worn, or other type of virtual display system.
In particular, a head-mounted display system is a virtual display system that is mounted on a user""s head-and projects an image for one or both eyes. Because a head-mounted display does not restrict a user""s movement, it offers a great potential for various practical uses, such as for viewing time and date, traffic and stock reports, or even e-mails. However, creating head-mounted displays typically involves tradeoffs between the following desirable factors: low weight, large field of view, large eye relief, large eyebox, and compact design. Ideally, one would prefer to have a head-mounted display that is no more intrusive than sunglasses and is capable of having a style desirable for consumers. The present invention provides a virtual imaging system suitable for forming, among other things, such an ideal head-mounted display.
The present invention offers a virtual imaging system that provides a user with an extended range of viewing (i.e., an enlarged xe2x80x9ceyeboxxe2x80x9d). The system allows a user to view a virtual image of an object field, which may be of a physical display or other objects. The system includes an imaging subsystem including at least one lens. The imaging subsystem is arranged such that its object field is at or near its focal point, thereby positioning the virtual image of the object field at or near infinity. In one embodiment, the imaging subsystem also comprises an image generator that is separated from the lens by approximately the focal length of the lens. The system further includes an eyebox spreader that is arranged to receive the light transmitted from the imaging subsystem and to redirect the light to a user""s eye. The eyebox spreader is adapted to effectively increase an eyebox of the imaging subsystem, i.e., the lateral range through which the user can see the complete virtual image. This eyebox spreading feature allows a user to more easily position himself to view the virtual image while at the same time allowing the virtual imaging system of the present invention to be compactly constructed and light in weight. The eyebox spreader requires that the virtual image of the imaging subsystem be positioned at or near infinity to project a clear image of the object field to the user""s eye.
Various embodiments of an eyebox spreader for effectively increasing an eyebox of an imaging subsystem are disclosed in accordance with the present invention. In one embodiment, an eyebox spreader comprises a Fresnel surface. A Fresnel surface defines an array of parallel, optically flat facets thereon. Light transmitted from the imaging subsystem strikes the facets on the Fresnel surface and is either reflected therefrom or transmitted (e.g., refracted) therethrough to be redirected to a user""s eye, while increasing its transverse xe2x80x9cwidthxe2x80x9d. A Fresnel surface may be provided on a thin substrate to form a Fresnel prism, or a Fresnel surface may be provided on a prism. Further, a plurality of Fresnel surfaces may be combined so that the facets of each Fresnel surface are offset from the facets of its adjacent Fresnel surface(s).
In a further embodiment, each of the facets on a Fresnel surface includes a first portion of a first reflectivity (50%, for example) and a second portion of a second reflectivity (100%, for example) to each form a beamsplitter. The first portions of the beamsplitters are adapted to partially transmit the light received from the imaging subsystem, while partially reflecting the received light toward a user""s eye to form a first series of wavefronts. The second portions of the beamsplitters are adapted to receive the light transmitted through the first portions of the beamsplitters and to at least partially reflect the received light toward the user""s eye to form a second series of wavefronts. The first and second series of wavefronts are alternately combined to form a contiguous wavefront. In other words, the second series of wavefronts fills in the gaps created by the first series of wavefronts, thereby eliminating dark gaps that the user""s eye may otherwise see.
The present invention thus discloses a method of spreading an eyebox of a virtual imaging system used for a user to view a virtual image of an object field. According to the method, a virtual image of the object field is imaged by the imaging subsystem at or near infinity, so that the wavefront of each object field point is planar, with a transverse width defined by the aperture of the imaging subsystem. Next, the wavefront is sequentially sliced into a plurality of light ribbons from the transverse width of the wavefront. Finally, the plurality of light ribbons are redirected toward a user""s eye so that the plurality of light ribbons will be separated along a collective transverse width of the plurality of light ribbons. The collective transverse width of the plurality of light ribbons is now greater than the transverse width of the original wavefront, thus the eyebox of the virtual imaging system is effectively increased.
In one aspect of the present invention, an eyebox spreader of a virtual imaging system may be configured to allow for light transmission therethrough. This xe2x80x9csee-throughxe2x80x9d eyebox spreader may be suitable for use in constructing a head-mounted display system including a display, so that a user can see the real world through the eyebox spreader while also being able to view a virtual image of the display thereon. In this case, a virtual image of the display will be superimposed on the real-world image.
In another aspect of the present invention, a virtual imaging system including a display may further include an eye view switch adapted for activating the display only when the user""s eye is viewing the display. In one embodiment, the eye view switch comprises an infrared light source, an infrared sensor, an infrared beamsplitter, and a dichroic beamsplitter. The light transmitted from the display is directed by the dichroic beamsplitter to the eyebox spreader, and then to the user""s eye. The infrared light transmitted from the infrared source is directed by the infrared beamsplitter and by the dichroic beamsplitter to the eyebox spreader, and then to the user""s retina. The infrared light reflected from the user""s retina reflects from the eyebox spreader and is directed by the dichroic beamsplitter and by the infrared beamsplitter to the infrared sensor. The display includes a plurality of view field points, and the infrared sensor includes a plurality of sensor positions. There is a one-to-one correspondence between each view field point of the display and each sensor position of the infrared sensor. The display is adapted to be activated when any of the sensor positions of the infrared sensor detects infrared energy reflected from the user""s eye, i.e., when the infrared sensor detects an eye view-angle directed to the display.
In a further aspect, a virtual imaging system of the present invention may be incorporated in a head-mounted virtual imaging system in the form of glasses to be worn by a user. The head-mounted virtual imaging system includes frames, and a virtual imaging system of the present invention mounted on the frames. As before, the virtual imaging system includes an imaging subsystem and an eyebox spreader. In one embodiment, the imaging subsystem includes a display located in the object field of the imaging subsystem, a display controller for supplying information to the display, and a battery for powering the display controller. For example, the display controller may provide information such as time, date, sensed data such as user""s pulse, stored data such as addresses, and notification data such as xe2x80x9ccell phone ringingxe2x80x9d.
By incorporating an eyebox spreader to effectively increase the eyebox, the present invention permits lowering the cost and also the size of a virtual imaging system. Further, the eyebox spreader redirects the virtual image in a way that conforms to the desired shape and form of eyeglasses, thereby improving design of head-mounted displays. Indeed, a compact, lightweight, and high-performance virtual imaging system of the present invention may ideally be used in a head-mounted virtual imaging system that xe2x80x9cwrapsxe2x80x9d around a user""s head, such as a head-mounted display system in the form of sunglasses.